Jump to content


  • Posts

  • Joined

  • Last visited


3,112 Excellent


Profile Information

  • About me
    Chartered Engineer

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

  1. A magnetic monopole in a magnetic field would be analogous to a charged particle in an electric field.
  2. Lunar regolith has a density of ~1500kg/m3, so a Starship buried to 3m would be under 4500kg per m2 or 7360Pa in lunar gravity. Internal air pressure of 1bar or 100kPa easily counterbalances that, though unintentional depressurisations and large unpressurized spaces would need additional consideration.
  3. Yeah, you don't really get shockwaves in a vacuum. It's how Galactica can take getting nuked In an actual orbital launch the heat shield will be very much further from the pad, and reflected energy goes with about r^4 if I remember my Scott Manley. Plus water deluge sound suppression system.
  4. Not latched doesn't necessarily mean it isn't deployed and producing power, but if it isn't secured that may cause issues in manoeuvres as I understand it.
  5. It won't. There's definitively too much still to do. Small chance of December. Far more likely next year.
  6. The way you equalise 4 strands is you equalise two adjacent strands by allowing them to float until the slack is taken up. O O X X By moments about an axis X-O, if the floating legs O and O are made the same then the fixed legs X and X are also the same, because X must equal O. I was doing a spell as an integration engineer on a job and the designers just before a deadline "helpfully" gave us a heavy component we had to cryo-fit flush underneath with no lifting features and refused to let us modify it. Had to lift it into a nitrogen bath with magnets, but the magnets individually weren't very capable (they could have been made stronger, but we'd start damaging the component). I had to do a deep dive into all the "de-rating" factors to see which ones could be made not applicable to the specific process. Other than non-equal legs, the other major de-rating factor for multiple legs is included angle between them. If they're pulling horizontally then capacity is being used on that and not vertical load. It's common for general purpose multi-legged lifting equipment to assume an included angle of 90-120 degrees, which de-rates each leg by the cosine of the half-angle. Magnets in particular do *not* like being pulled sideways though. They'll slide until the pull is mostly vertical, except now that's a smaller lifting footprint so the load probably becomes unstable, the lifting surface angles to the horizontal, and then the magnets slide again until the load is dropped. So if you're only using a very small included angle on the legs anyway to prevent sliding, that de-rating factor also drops out almost completely. Also had to account for cryo-conditions as well. All-stainless steel equipment, and samarium-cobalt magnets that would only lose ~10% of their magnetism unlike neodymium one which become basically useless at LN2 temperatures. Got some beautifully shiny stainless steel D-shackles that became prized office paperweights after fulfilling their function. And then the magnet supplier's courier let us down, so I had to get the magnets taxi'd across the country overnight on my own buck to make the start of testing the next day. Fun little job.
  7. Without taking sides in the topic overall, SpaceX just has a better PR department. Their streams are better hosted and in better quality than NASA TV, and they do a lot of their activities in public where we can watch the progress step-by-step in live HD as it happens rather than in still frames weeks after the event after security has cleared the photos for ITAR. Also in the areas of human spaceflight and rocketry (arguably the coolest areas) SpaceX is free to innovate its products to produce things that are modern and exciting, whereas theses days NASA is not. Now that's not an entirely fair comparison, but it is why it's easy to be a fan of SpaceX.
  8. A kilometer-long tether would strain about 3m per strand under load, so that's a pretty effective load-equaliser even without resorting to a tension equaliser, which I have personal experience of designing for an industrial 4-strand lifting application. More strands is just more equaliser complexity, but the equaliser's probably unnecessary. The day/night cycle is an interesting point, but what matters is the rate of change. 7.5m is only a big deal if it takes just seconds to equilibriate, which sounds unlikely.
  9. A gradual start would not produce shocks of the order 1g. Particularly not with a tether which, as you've pointed out, is reasonably elastic. Roughly 3m extension over 1km for a 20x 16mm diameter strand ribbon for 3x Starships as I suggested calculated above. Yes, there may be some small oscillations. Tangential velocity at 1g at 1km is ~70m/s. Assume the spacecraft has a 1 deg deviation from tangent and hits the end of the tether. Radial velocity is ~1.2m/s. Kinetic energy is ~187kJ. Extension is ~ 0.7m. Peak acceleration is ~0.23g. That's a very big oscillation because we're starting from full speed. Very likely oscillations would be damped out by the time it got up to speed. Most would occur as the slack in the tether is taken up at under 1 tenth that velocity. A small damping shock absorber and a brake are sufficient. The motor doesn't need to see any dynamic load at all.
  10. Please explain? The tether can be paid out slowly and then gently braked to a stop. No excessive acceleration required. Start up of the rotation is as gentle as the thrusters that initiate the spin up manoeuvre. This is generally very much less than 1g. A Dragon capsule masses ~15t. A Draco thruster provides 0.04tf thrust. Even as a combination together, this is very much less than 1g. There's no need to do anything quickly. In fact falling objects is a very good reason not to.
  11. There's no reason a gradual and coordinated startup would produce any sort of whip.
  12. No it absolutely shouldn't. Conservation of angular momentum would spin a 2 starship system up to over 60RPM as the tether retracted. 2 Dragons would be much worse due to the closer finishing position - over 10,000RPM. The system needs to be stopped for deployment and retraction, in which case the winch motor only needs to be strong enough to make the tether coil up neatly (which is not zero force - wire rope prefers to be straight). Also I'd also say the tether is not such a significant mass that having a lighter one would be significantly less propellant expended for recovery. But it is relatively trivial to have double or triple redundancy. Quickly get to a situation where failure is not credible. We regularly trust lives to wire rope systems. Elevators, Cranes. It's not a big deal. The only novel factors for space travel are mass budget (which is becoming less important), and vacuum operation, which is just double checking material properties of the tether are appropriate.
  13. A standard steel kilometre long 1x19 wire rope 10mm in diameter would mass half a ton. It could sustain a minimum breaking load of 10t at each end, of which self-weight would contribute 0.125t. With a safety factor of just under 2, there could be a 5t module at each end, a 10.5t spacecraft total. Alternatively a quad-tether of such wire ropes could sustain 2x10t modules with multiple redundancy for 22t. Tethering two 250t loaded Starships together would take a ribbon of 20 16mm 1x19 wire ropes for a safety factor of 2, and would be very tolerant of individual rope failures. They'd weigh 25t (total spacecraft weight 525t). Stronger ropes are available that would reduce the number of strands required at the cost of less redundancy, but flexibility and compactness of the reeling mechanism becomes an issue with larger diameter ropes. Although very large amounts of redundancy are probably unnecessary - multi-strand wire ropes are already self-redundant, and if any failures are detected you'd just reduce the speed of rotation of the combined craft. It's also quite unlikely that a full 1g is required for crew comfort. So tethers are actually quite easy with standard engineering materials if you have the mass budget for the mechanism.
  14. I had a really vivid dream last night (pretty rare for me) that Superheavy launched, cleared the tower, but came down on the beach moments later. Coming on here in a still half-awake state to reassure myself that no, none of that was real, it wasn't even stacked with starship let alone licensed for a launch. (Also I live in the UK, not Texas. There is zero chance I'd be walking around the aftermath). The subconscious can be a strange place sometimes.
  • Create New...